1. Field of the Invention
This invention deals with multiple quantum well photodetectors intended for unpolarized light detection.
2. Description of the Related Art
A variety of photodetectors have been developed that are useful for focal plane arrays and other applications. They may generally be classified as intrinsic semiconductor, extrinsic semiconductor and multiple quantum well devices. Each type of device, however, has certain limitations to its usefulness.
Photodetectors made from intrinsic semiconductor materials face severe material stability and produceability problems, especially if they are used to detect radiation with a wavelength greater than 10 microns. Current technology requires the use of HgCdTe to achieve an infrared detection capability, which is highly desirable. An example of this type of photodetector is discussed in Reidel, et al., "High Performance Photovoltaic Infrared Devices in Hg.sub.1-x Cd.sub.x Te on Sapphire", Applied Physics Letters, Vol. 46, No. 1, Jan. 1, 1985, pages 64-66. While mercury (Hg) is highly volatile, it is necessary to add more and more mercury to the semiconductor to obtain narrow band detection. It is quite difficult to control the mercury in many applications, resulting in non-uniform detection pixels.
Intrinsic semiconductor photodetectors generally exhibit large bandgaps, and thus can detect only high energy photons; low energy photons essentially see a transparent structure. Extrinsic semiconductor photodetectors add a dopant, such as gallium or arsenic for a silicon photodetector, to reduce the bandgap and achieve sensitivity to low energy photons. However, an optimization of detector sensitivity to specific wavelengths is restricted because of a limited availability of allowable dopant species. Furthermore, in practice it is difficult to establish and maintain the current dopant level, and to keep the dopant pure.
A different photodetector that offers more flexibility than semiconductor detectors is the one-dimensional multiple quantum well. In this type of device, the operating characteristics are controlled by the width and height of the wells, rather than by selecting from limited available materials. A multiple quantum well detector can be very thin, on the order of a micron or less, making it much more radiation hard than either intrinsic or extrinsic semiconductor detectors.
Multiple quantum well detectors are formed from superlattice stacks of ultrathin semiconductor layers, typically Group III-V semiconductors. With these materials the energy bandgap is direct, permitting light to be efficiently emitted or absorbed without the aid of lattice vibrations. Input photons transfer energy to electrons in the well, exciting the electrons from a ground state, while an electric field moves the electrons laterally. The materials are characterized by large charge carrier mobilities, and are easily doped with impurities. They can form solid solutions of various proportions with identical crystal structures and well-matched lattice parameters, but with different energy bandgaps and indices of refraction.
One type of superlattice is the "doping superlattice", which is obtained by periodically alternating n and p doping during the growth of an otherwise uniform semiconductor such as gallium arsenide. A basic discussion of this n-i-p-i structure is given in an article by Klaus Ploog and Gottfried H. Dohler, "Compositional and Doping Superlattices in III-V Semiconductors", Advances in Physics. Vol. 32, No. 3, 1983, pages 285-359. This article presents a general discussion of n-i-p-i structures, as well as the spatial control of optical absorption by a voltage pattern applied to the n-i-p-i structure. Other articles which describe specific quantum well structures are:
Choi et al., "Multiple Quantum Well 10 .mu.m GaAs/Al.sub.x Ga.sub.1-x As Infrared Detector With Improved Responsivity", Applied Physics Letters. Vol. 50, No. 25, 22 June 1987, pages 1814-16.
Levine et al., "High-Detectivity D*=1.0-.times.10.sup.10 cm/Hz/W GaAs/AlGaAs Multiquantum Well .lambda.=8.3 .mu.m Infrared Detector", Applied Physics Letters, Vol. 53, No. 4, 25 July 1988, pages 296-98.
Levine et al., "Bound-To-Extended State Absorption GaAs Superlattice Transport Infrared Detectors", Journal of Applied Physics. Vol. 64, No. 3, 1 August 1988, pages 1591-93.
While one-dimensional multiple quantum well photodetectors made of heterojunction material provide flexibility in performance optimization for long wavelength infrared detection, especially in the greater than 10 micron wavelength range, the quantum efficiency of these devices is limited for unpolarized light detection. This is because these devices are not sensitive to optical polarization parallel to the detector plane. To excite an electron in a well, the electric field associated with the photons must be perpendicular to the vertical barrier walls (in the first order).
Ideally, the light to be detected would be directed onto the detector at 90.degree. to the detector plane, to obtain the best image. However, since the plane of polarization for a light beam is normal to its direction of propagation, this would place the polarization plane parallel to the detector plane, so that it could not be detected. To compensate for this, the light is normally directed onto the detector at an angle to the detector plane which is sufficiently small so that a substantial component of the polarization is perpendicular to the detector plane and thus absorbed, but at an angle great enough to preserve adequate image clarity. In practice, an incident angle of about 45.degree. has been used for this purpose.
Since the component of the incoming beam having a polarization parallel to the detector plane will not be absorbed by the multiple quantum well detector, an inefficiency is built into the system. As the angle of the beam to the detector plane increases, a thicker detector is necessary to retain even the same partial level of absorption. However, thick detectors are undesirable because they are less radiation hard, and require a higher operating bias voltage.